with the editors available bg2 is a close second on my list of games with longevity (myth being first, having played that for more than 4 years, and i hate first person shooters).

i highly recommend getting the first in the series. besides it being a great game in itself and filling in the background, it will let you familiarize yourself with the controls, game mechanics, and items as it is a bit less complex than bg2. also, since you can import your character into bg2, you have the opportunity to completely control who and what you are (not to mention boost your stats beyond normal starting allowances). you can probably find it for less than $10.